type as plugin API.
This should be called in `class_init` of elements that expose new types (i.e. enums, flags or internal GObjects) via properties, signals or pad templates.
Types exposed by plugins are not automatically added to the documentation as they might originate from another library and should in that case be documented via that library instead.
By marking a type as plugin API it will be included in the documentation of the plugin that defines it.
a set of PluginAPIFlags to further inform cache generation.